In some valve drive mechanisms in which a valve 52 is opened and closed by a rocker arm 50 as shown in FIG. 4A, a shim 55 having a predetermined thickness selected from a plurality of ranks of shims is interposed between a pad surface 51 of the rocker arm 50 and a stem end surface 53 of the valve 52 to adjust a valve clearance. The plurality of ranks of shims have respective plate-like portions with thicknesses that differ by several tens of μm (Patent Literature 1). The shim 55 includes a plate-like portion 56 and a tubular portion 57 extending downward from the peripheral portion of the plate-like portion 56. The valve 52 is fitted into the tubular portion 57 from below so that the stem end surface 53 contacts the lower surface of the plate-like portion 56, which causes the pad surface 51 to contact a shim top surface 58 which is the upper surface of the plate-like portion 56.
The shim top surface 58 is prepared by a variety of methods. In the case where the shim top surface 58 is prepared by cutting, the shim 55 is set onto a lathe, and rotated about a center X of the shim top surface 58. A blade is applied to the shim top surface 58 to perform cutting. At the center X of the shim top surface 58, the peripheral speed is 0 even during rotation, and thus cutting cannot be performed. Therefore, a center hole 59 has been formed at the center X before the cutting. The center hole 59 does not disappear but remains even after the cutting, and an intersection line a between an inner peripheral surface 60 of the center hole 59 and the shim top surface 58 defines a corner portion of an opening edge of the center hole 59.
The shim top surface 58 has been crowned with its center bulged upward by a difference in height of about 1 to 20 μm as shown in FIG. 4B for the purpose of preventing corner contact due to misalignment with respect to the rocker arm 50, that is, a phenomenon that a corner of the pad surface 51 contacts the shim top surface 58 when the rocker arm 50 is tilted. This crowning is also intended to facilitate cutting.
Similarly, the pad surface 51 facing downward has also been crowned with its center bulged downward by a difference in height of about 1 to 30 μm in the width direction, and with its center bulged downward by a radius of curvature of about 10 to 20 mm in the longitudinal direction.
In FIGS. 4B and 4C, the curves of the shim top surface 58 and the pad surface 51 are shown as exaggerated compared to the actual curves for clarity.